Electric signaling system



March 7, 1944. F. FAIRLEY ETAL 2,343,759

ELECTRIC SIGNALING SYSTEM Filed June 1'7, 1942 2 Sheets-Sheet 1INVENTORS March 7, 1944. F. FAIRLEY ETAL 2,343,759

ELECTRIC SIGNALING SYSTEM Filed June 17, 1942 2 Sheets-Sheet 2 L A N3 RC/%% 07 F T N3 NI N2 W A A E R2 i /r B 0 V|/A(7'0/?I 5y a, I AM I ,7!ATTORNEY Patented Mar. 7, 1944 mac'rmc SIGNALING srs'riim Frank Falrleyand Raymond Walsh, London, England, assignors to International StandardElectric Corporation, New York, N. Y.

Application June 17, 1942. semi N... 441,394 In Great Britain September15, 1941 12 Claims.

The present invention concerns selective electric wave receivers and-inparticular those used for receiving ringing and dialing signalstransmitted over telephone communication circuits.

In voice frequency ringing systems used in communication circuits,difllculty has been encountered in the past in preventing the accidentalgenerally termed talking up.

Various devices have been proposed and used hitherto to avoid thisdifficulty. Such devices are often complicated, involving guard circuitarrangements with critically adjusted relays for preventing theoperation of the ringer while speech is being received, and sometimesemploying more than one ringing frequency. Such arrangementsare eventhen not always completely effective in preventing false operation.

One relatively simple arrangement for avoiding false operation of theringing arrangements by voice currents is described in United StatesPatent No. 2,117,835. This makes use of a silver sulphide resistancewhich is also used in carrying out the present invention. Thearrangement of the above mentioned patent operates by virtue of the highvoltage of the ringing currents which causes the silver sulphideresistance to present negligible impedance to the ringing currents; butat the low speech voltages the impedance is so high that any currentswhich can reach the ringer are made so small as to be unable to affectit. The present invention is an improvement on this arrangement butoperates on an entirely different principle and has a wider application.Discrimination between the voice and ringer currents is obtained invirtue of the different frequency distribution of energy of thesecurrents and does not depend upon nor require a large voltagedifierence. In the arrangement of this invention, not only do the voicecurrents fail to operate the ringer, but in addition positive meanscomes into operation actively to prevent it from responding except whenthe particular ringing currents are received.

The object of the present invention is to provide a simple and reliablesignal receiver in which talk ing up is effectively prevented withoutthe use of extra frequencies or guard circuits or relays. While itsprimary application is to ringing or dialing arrangements in connectionwith transmission systems, it is not limited to such arrangements but isapplicable much more broadly to any cases where operation is desired byincoming signal waves having some particular type of frequencydistribution of energy, and where operation must not occur when theincoming waves have some other type of distribution.

The invention in its preferred embodiment depends for its eifect on anarrangement whereby the incoming signals do not themselves operate thereceiver, but cause local oscillations to be generated for this purpose,and then only if the frequency distribution of the incoming energy issubstantially of some predetermined forms In the case of particularinterest, the incoming energy which is to operate the receiver will bepractically of a single frequency, but it might equally well consist ofseveral single frequencies, or bands of frequencies. When the incomingenergy has some other distribution, no oscillations can be produced andgenerally the effect will be further to inhibit any tendency tooscil1ation.

While talking up on speech is extremely unlikely with the circuit of theinvention, it is conceivable that that might happen if the speech shouldmomentarily consist of practically only the ringing frequency at highlevel. Even in this unlikely event operation of the receiver isprevented by the use of temperature dependent resistance elements suchas thermistors or lamps whose response to changes .is relatively slow,in a manner to be fully explained below.

In view of the slow response of the thermistors, the circuits of theinvention would not receive dialing impulses satisfactorily. Therefore,in order to be able to take advantage of the invenion in a dialingreceiver so that it shall not be subject to risk of talking up, a prefixsignal would be used when dialing which signal would operate the deviceas described, causing suitable means to function to condition thereceiver so as to enable it to receive the impulses for the duration ofthe dialing, after which it would revert to the safety condition.

According to one feature of the invention, a selective electric wavereceiver comprises means for comparing the amount of received energycontained within certain ranges of frequencies with tire amount ofreceived energy contained within certain other ranges of frequencies;and

means to cause the operation of a signal receiv- According to theinvention, also, the signal receiving device may be operated by localoscillations which are caused to be generated on the receipt of signalshaving a predetermined frequency distribution of energy, suchoscillations being prevented when signals with some other energydistribution are received: and slow act-.

ing, thermally operated means may be provided for discriminating betweensignals having a predetermined energy distribution and those having someother energy distribution.

The invention will be better understood from the following detaileddescription read in conjunction with the accompanying drawings in whichFigs. 1 and 2 are block schematic diagrams used for explaining theprinciples of the invention: and

Figs. 3 and 4 are schematic circuits of preferred embodiments thereof.

In the circuits shown in the above mentioned Figs. 3 and 4, valves areindicated as triodes with indirectly heated cathodes, the heatingcircuits not being shown in the interests of clearness. The invention isnot limited to the use of such valves, and pentodes, or any otherconvenient types, may be used if preferred. A battery is shown by theconventional symbol to indicate the plate potential source, althoughthis should not be taken to mean that the plate circuits are necessarilysupplied in this way, nor that any particular voltage is implied. Thearrangements for biasing the control grids are shown in the form of theusual condenser-shunted resistance connected in series with the cathode,but this is likewise not an essential arrangement. It is to beunderstood that circuits operated in accordance with this invention maybe provided with any suitable valves, and that appropriate arrangementsmay be supplied in any of the well known ways.

Figs. 1 and 2 are block schematic circuit diagrams which will be used toexplain the principles involved. Fig. 1 represents the circuit of asignaling receiver suitable, for instance, for receiving the ringingcurrents in a telephone transmission system employing voice frequencyringing. The ringing currents are applied from the line L, and afterpassing through the receiver, cause the operation of a relay system ofsome suitable known type indicated at R.

The signals are applied to an amplifier AI and the relay system R isoperated from an amplifier A2 which is provided with a feed back paththrough a Wheatstone bridge network W. This network has four branches ofwhich two comprise the constant resistances RI and R2 and the other twocomprise resistances TI and T2 which are variable with the temperature.TI and T2 are respectively provided with electrically insulated heatercoils operated from the amplifier Al. One pair of diagonal terminals ofthe network W is connected to the input of amplifier A2 and the otherpair of diagonal terminals is connected to the output thereof. Thenetwork W accordingly provides afeedback path for the amplifier.

The resistancesTl and T2 are preferably composed of material (such forinstance as silver sulphide) whose resistance varies with thetemperature and may be ,mounted in containing envelopes Trl and Tr! withthe corresponding heater coils as indicated in Fig. 1. In this form theymay constitute elements known as thermistors of the indirectly heatedtype, of which the resistance depends mainly on the current flowingthrough the heater coil and practically not at all on the currentflowing through the resistance element itself. Such thermistors have anegative temperature coefficient of resistance.

The heaters are each connected through a corresponding network NI or N2to the output of amplifier Al. Any currents arriving from L will beamplified and will flow through the heaters of Trl and T11 after beingattenuated' in the networks NI and N2. The input circuits of thesenetworks are shown in Fig. 1 connected in parallel. They could equallywell be connected in series if preferred.

Now let it be assumed that there are no signals coming in from the lineand that the values of the resistances RI, R2 and Ti, T2 in theWheatstone bridge network are so chosen that the feedback produced inthe circuit of amplifier A2 is negative. It will then be quiescent. Nowassume that a ringing signal comes in, consisting, for example, of asingle frequency applied continuously, and suppose also that it passesthrough the network NI substantially without attenuation, but that it isblocked by network N2. The heater of Trl will then receive a current andwill heat the corresponding resistance Tl, but TrZ will be practicallyunaffected. The result will be to decrease the resistance of TI. Nowassume that when the temperature of Trl has become constant, theresistance Tl has so changed the condition of the bridge network thatthe feedback has become positive. The amplifier A2 may then be caused togenerate oscillations at a suitable frequency determined, for example,by a tuned circuit appropriately located in the amplifier, whichoscillations can be used to operate the relays in R by a suitablerectifying circuit (not shown) located in R. Thus the effect of applyingringing current to the input of amplifier Al is to cause (indirectly)the operation of the relays in R as desired. When the ringing current isremoved the thermistor Trl will cool down and in a short time thefeedback will again become negative and the oscillations operating Rwill cease. It is preferable, though not strictly necessary, that thefeedback introduced by the Wheatstone bridge network should be negativewhen no signals are being received: it may however be zero, or evenpositive so long as the amount of feedback is insufficient for there tobe any risk of the amplifier oscillating.

In general, the ringing receiver will be bridged across the line so thatit will be subjected to the ordinary speech transmission currents aswell as to the ringing currents. T ese speech currents cover a band offrequencies perhaps 2700 cycles wide or more with a certain average typeof energy distribution. The ringing frequency will of course occur, butthe energy associated therewith will generally be small compared withthe total energy in the band. In Fig. 1 if the network N2 were sodesigned that it allows substantially the whole band to pass except theringing frequency, and if NI were so designed that it blockssubstanassa'no is applied. Knowing the nature of DI and m it tially thewhole band except the ringing frequency, then heater of Trl will receivea very small amount of energy due to the ringing frequency component ofthe speech band, but that of Tr2 will receive a large amount of energydue to nearly the whole band. The result will be that the resistance ofTI will be slightly reduced but that .of T2 will be reduced much more.The effect can be made to change the bridge network m such a manner thatthe feedback produced becomes more negative than before, so that theamplifier A2 is taken further away from the oscillating condition and soremains quiescent, the relay system B being unaffected. The speech ofthe present invention and that of the United States Patent No. 2,117,835referred to above, may be seen in the fact that the circuit of theinvention may be designed so that if the incoming currents contain anyadditional currents outside the predetermined frequency or band offrequencies, the relay device R is still prevented from operating by thepresence of these outside frecurrents have thus operated to prevent therequencies although the predetermined frequency,

sponse of the relay system R.

The networks Ni and N2 could for example be respectively complementaarynarrow band pass and band elimination filters, the narrow band beingcentral round the ringing frequency.

The thermistors Trl and Tr2 are relatively slow to operate and theirtemperature is chiefly determined by the instantaneous energy in thecorresponding heater integrated over a period of time, which iscontrollable by suitable design, and which can be made long comparedwith the period or duration of any of the frequencies in the speechwave. Thus even if the speech energy should consist momentarily of onlythe ringing frequency at an abnormally high level (a condition which inmany voice frequency ringing systems causes false ringing), its effecton the thermistor Trl will be negligible as it will not have had time'torespond. On this account the system of the invention is entirely proofagainst talking up.

In order to make clear the working of the invention, a particular(though not unlikely) series of circumstances has been assumed in theabove explanation. The invention is however of a much broader nature,and is not necessarily applicable only to ringing systems. As an exampleof another possible arrangement, a dialing system might be operated overthe line, employing two voice frequencies simultaneously applied 45thereto, and the network NI and N2 might then consist of band pass andband elimination filters having two bands corresponding to the twofrequencies. It might also be assumed that the voice frequency circuitis used sometimes for picture 0 transmission, in which case the energydistribution in the band would be quite different from that for ordinaryspeech. This is therefore another case in which the receiver mustoperate when it receives energy having one kind of diso5 tribution andmust not operate when it receives energy of another kind ofdistribution; stated in this way the broadest aspect of the invention isexpressed.

In order to appreciate how the receiver of the no a function of El/E2.The values of El and E2 are determined by the networks Ni and N2 and bythe energy distribution of the band of frequencies applied to the inputterminals. In order that the 70 receiver shall operate for adistribution DI and shall not operate for a distribution E2, the ratioTl/T2 must change in a manner tending .to make the feedback positivewhen'Dl is applied, and

must change in the opposite manner when D2 75 or band, is there also. Inthe arrangement of Patent No. 2,117,835 the relays will always operatewhen the predetermined frequency is present with suiiicient voltage,irrespective of what other frequencies are also there; it is accordinglyunable to detect the presence of extra frequencies.

By a modification of the arrangement of Fig. 1 a proportion of theoutput of the amplifier A2 may be fed back to the input of amplifier AL.

If the oscillation frequency of A2 be chosen so that it will be passedby the network NI, and

excluded by N2 then the arrangement may be made to lock itself when onceoperated. Thus if the ringing current be applied long enough to startthe oscillations of amplifier A2 those oscillations will-be fed throughthe amplifier Al to the heater of TH and can be made to maintain itstemperature at or near the temperature produced by the incoming ringingcurrents, so that the feedback is kept positive. The relays in R willthen remain operated until the arrangement is broken down by independentmeans (such as by momentarily cutting either of the feedbackconnections) The feedback connection between the amplifiers A2 and AImay be made in any convenient way.

More generally, the oscillating frequency and the networks NI and N2 maybe so chosen and designed that when som of the energy of theoscillations is fed back in the manner just described, and after ringingcurrent has been applied, the ratio Tl/T2 is so modified that thefeedback produced by the network W either is made more positive, or isunaltered, or is made less positive, than it otherwise would have beenif the applied currents acted alone. In the first case the arrangementmay look itself permanently in operation; in the last case it may bemade to operate intermittently while the ringing currents are applied,by suitable choice of the conditions, this being possible as a result ofthe relatively slow response of the thermistors. In the latter case,also, all operation ceases a short time after removal of the ringingcurrents. In the case when the arrangements are such that the feedbackintroduced by W is not affected by the oscillations, the time of therelease of the relay system It will be prolonged due to the energy ofthe oscillations which will continue to fiow with diminishing amplitudein the bridge, after removal of the ringing current.

In Fig. 2 is shown another arrangement employing only one amplifier A.The incoming signals are applied from the line L to the input of thisamplifier which is provided with a feedback path through a Wheatstonebridge network W similar to that described in connection with Fig. 1.The heaters of the thermistors Trl and Tr2 and in this case connectedthrough the networks NI and N2 to the output of the amplifier A, and inthis case an additional network N3 is interposed between the amplifieroutput and the relay system R. This network is necessary to ensure thatthe relay system B shall be operated by the cally generated oscillationsand not directly by the incoming signal.

With this arrangement, feedback is produced, as before, through thenetwork W, which will be so adjusted that initially this feedback isnegative, or at least insufiiciently positive for oscillations tooccur). If single frequency ringing currents be received, they will beamplified by the amplifier A and will thereafter reach the inputcircuits of the networks N I, N2 and N3. If N2 and N3 be designated toexclude the ringing current and N i to pass it, then as in the case ofFig. l the heater of Trl will receive energy and that of Tr2 willreceive practically none. Arrangements are made so that thecorresponding reduction of the resistance Tl causes the feedback tobecome positive thereby initiating oscillations, the frequency of whichis chosen so that they can pass the network N3 and operate the relaysystem R. If it be assumed that the oscillations are also excluded by NIand N2 then the circuit behaves as in the case of Fi 1 without theadditional coupling between th amplifiers Al and A2.

The networks NI and N2 will preferably be designed as before so that Nisubstantially excludes the band of speech frequencies and N2 passes it,and so that th ratio TI/T2 changes to make the feedback more negativewhen speech currents are being received.

With this circuit, the network N3 should preferably be designedsubstantially to exclude the speech currents in order that the relaysystem R may not be in any danger of being operated directly by them. Aconvenient though not essential arrangement would be to choose thefrequency of oscillation so that it lies outside the band of speech andringing frequencies. NI could then, for example, be a relatively simplelow pass or high pass filter.

By a modification of Fig. 2, the variable resisterably exclude thespeech currents from R to prevent it from being directly operated by thespeech currents.

Theoperation of the circuit of Fig. 2 and the modification thereof havebeen explained for clearness in terms of a particular case of a linetransmitting ordinary speech and using a single frequency ringingsystem. As in the case of Fig. 1, these circuits are not limited to sucha particular system, but may be designed to discriminate between twosystems of currents comprising bands of frequency with different energydis. tributions DI and D2, by proper design of the discriminatingnetworks, according to the principles explained above in connection withFig. 1. It will also be noted that the oscillations can be made toaifect the resistances of the elements TI and T2 of the bridge W, eitherdirectly, or through the heaters, provided that their frequency and thediscriminatingnetworks are so chosen and designed that a proportion ofthe energy of the oscillations is allowed to how through one or boththermistors.

Thus, as in the case of the modification of Fig.

l in which feedback is provided between the amplifiers Al and A2, theoscillations may be made to aifect the bridge so that the feedback tendsto be made more positive, or less positive, or is ances Ti and T2 are ofself heated type, and are shunted respectively by discriminatingnetworks MI and M2, the networks NI and N2 shown in Fig. 2 beingremoved. Ti and T2 may for example be directly heated thermistors: thatis.

they may consist of silver sulphide resistance elements not providedwith heating coils, but designed to be heated by the current flowingthrough them. Ti and T2 might also consist of other types of elementwhose resistance depends upon the current flowing through them, such aslamps, and may have a positive or a negative temperature coeflicient ofresistance. The networks MI and M2 will be two terminal impedancesdesigned to shunt the elements TI and T2 in a selective manner. All theother elements will be shown in Fig. 2 and operate in the same way. Thusif it be assumed, as before, that single frequency ringing currents areused, then (the bridge network W having been adjusted to producenegative feedback initially when no currents are being received), Mi andM2 might be designed so that the ringing current passes through TI, butsubstantially none of it through T2, thereby making the feedbackpositive; and so that the speech currents fiow through T2 butpractically none through Tl making the feedback more negative;oscillations for operating R being produced in the first case but not inthe second. Network N3 as in the case of Fig. 2 should prefunaffected.In the first case the circuit may be made to lock in operation afterapplication of the ringing currents, and in the second case intermittentoperation can be made to occur durinl the period of application of theringing currents. Figs. 3 and 4 show the circuits of two preferredembodiments of the invention according to Figs. l and 2 respectively, inwhich corresponding elements are given the same designations; and as theoperation of such elements has already been fully explained they willnot be again described in detail.

In Fig. 3,,the amplifier Ai comprises a thermionic valve VI providedwith input and output transformers ITI and OTi.respectively. Incomingsignals are applied from the line to the input terminals A, B connectedto the primary winding of the transformer ITI, and the secondary windingof the transformer OTi is connected to the networks N I and N2. Theamplifier A2 likewise comprises a thermionic valve V2 with input andoutput transformers IT2 and GT2 respectively. The diagonals of theWheatstone bridge network are connected respectively to the primarywinding of the transformer IT2 and to a secondary winding of 0T2. Therelay system R is connected to another secondary winding of 0T2.

A condenser CI is shown connected across the first mentioned secondarywinding of 0T2 for the purpose of providing a tuned circuit for fixingthe oscillation frequency of the valve V2. Alternatively, Cl could beconnected across the primary winding of IT2; and any other conline toinput terminals A and B connected to the primary winding of IT. One pairof diagonal ferminals of the Wheatstone bridge network W is to theopposite branches of the bridge network connected to the control gridcircuit of the valve V in series with the secondary winding of IT, Theother pair of diagonal terminals is connected to the secondary windingof OT in parallel with the input circuits of the networks NI, N2 and N3.Feedback is thus directly obtained between the plate and grid circuit ofthe valve. The circuit operates exactly as explained in connection withFig. 2. Alternative arrangements may be obtained by interchanging thepositions of the bridge W and the secondary winding of the inputtransformer IT so that the latter is directly connected to the controlgrid of the valve 'and the former i connected to ground; also theconnections between the bridge W and the control grid circuit may bemade through an additional input transformer the secondary winding ofwhich is connected in series with that of IT: other like arrangementsare clearly possible. The circuit of Fig. 4 may also be modified inaccordance with the modified arrangement of Fig. 2 whereby the networksNI and N2 are omitted, and directly heated thermistors (or lamps)shunted by impedance networks MI and M2 are used, exactly as previouslydescribed.

Although the invention has been explained in terms of specificembodiments, it is not intended to be limited thereto; and various otherarrangements in accordance with the principles ex-' plained will occur.to those skilled in the art. i

What is claimed is: i 1. A selective electric wave receiver comprisingfilter networks of unlike frequency outputs, output connections from therespective filter networks to the opposite branches of a Wheatstonebridge network including resistances having neg- -ative temperaturecoefficient of resistance, and

a signal receiving device connected to the output side of the bridgenetwork and said receiving device being operative when the ratio ofenergy of one branch circuit to the other is greater than a given value.

2. A selective electric wave receiver comprising filter networks ofunlike frequency output, output connections from the respective filternetworks to the opposite branches of a Wheatstone bridge networkincluding thermistor resistances having negative temperature coefiicientof resistance, an amplifier connected to the opposite branches of thebridge network to establish a feedback circuit thereto and a signalreceiving device connected to the output side of the amplifier and saidreceiving device being operative whenthe ratio of energy of one branchcircuit to the other is greater than a given value.

3. A selective electric wave receiver comprising a plurality of filterdevices of unlike frequency output, output connections from therespective filter devices to the opposite branches of a Wheatstonebridge network including slow acting thermistor resistances havingnegative temperature coefiicient of resistance, an amplifier connectedto the opposite branches of the bridge network to establish a feedbackcircuit thereto and a signal receiving device connected to the outputside of the amplifier.

4. A selective electric wave receiver comprising an amplifier, filternetworks of unlike frequency characteristics connected to the output ofsaid amplifier, output connections from the respective filter networksto the opposite branches of a Wheatstone bridge network includingthermistor resistances having negative temperature coefficient ofresistance, a second amplifier connected to establish a feedback circuitthereto and a si nal receiving device connected to the output side ofthe second amplifier and said receiving device being operative when theratio of energy of one branch circuit to the other is greater than agiven value.

5. A selective electric wave receiver comprising an amplifier, filternetworks of unlike frequency output, output connections from therespective filter networks to the opposite branches of a Wheatstonebridge network including indirectly heated thermistor resistances havingnegative temperature coefiicient of resistance, a second amplifierconnected to the opposite branches of the bridge network to establish afeedback circuit thereto and a signal receiving device connected to theoutput side of the second amplifier and said receiving device beingoperative when the ratio of energy of one branch circuit to the other isgreater than a given value.

6. A selective electric wave receiver comprising an amplifier, filternetworks of unlike frequency output connected to the output of theamplifier, output connections from the respecan amplifier, a pair offilter networks of unlike frequency output connected to the output ofthe amplifier, a Wheatstone bridge network including thermistorresistances having negative temperature coefiicient of resistance,output connections from the respective filter networks to heater coilsof the thermistor resistance, an amplifier connected to the oppositebranches of the network to establish a feedback circuit thereto and asignal receiving device connected to the output side of the secondamplifier.

8. A selective electric wave receiver comprising an amplifier, filternetworks of unlike frequency output connectedto the output of the saidamplifier, a Wheatstone bridge network including thermistor resistanceshaving negative temperature coeificient of resistance, outputconnections from the respective filter networks to heater coils of thethermistor resistance, a second amplifier connected to the oppositebranches of the bridge network to establish a feedback circuit thereto,said second amplifier having frequency oscillation range correspondingto the frequency output of one of the filter networks and a signalreceiving device connected to the output side of the second amplifierand responsive to the operative when the ratio of energy of one branchcircuit to the other is greater than a given value.

9. A selective electric wave receiver comprising a thermionic valveamplifier having associated input and output transformers, filternetworks of unlike frequency output connected to the secondary of saidoutput transformer, a Wheatstone bridge network including thermistorresistances having negative temperature coefficient of resistance, asecond thermionic valve amplifier having associated therewith input andoutput trans-- formers, the latter having its primary winding connectedin series with the plate circuit of the first thermionic valve andhaving a secondary winding connected to the first pair of diagonals ofthe bridge network and said latter input transformer having a primarywinding connected to the input of the receiver and having a secondarywinding connected to the control grid circuit of said second valve andin series with the second pair of diagonals of the bridge network, and asignal receiving device connected to another secondary of the latteroutput transformer.

10. A selective electric wave receiver comprising a thermionic valveamplifier having associated input and output transformers, filternetworks of unlike frequency output connected to the secondary of saidoutput transformer, a Wheatstone bridge network including thermistorresistances having negative temperature coefficient of resistance, asecond thermionic valve amplifier having associated input and outputtransformers, connections from the diagonals of the bridge networkrespectively to the primary of the latter input transformer and to thesecondary of the latter output transformer and a signal receiving deviceconnected to another secondary of the latter output transformer.

11. A selective electric wave receiver comprising a thermionic valveamplifier having associated input and output transformers, filternetworks of unlike frequency output connected to the secondary of saidoutput transformer, a Wheatstone bridge network including thermistorresistances having negative temperature coefficient of resisting athermionic valve amplifier having associated input and outputtransformers, filter networks of unlike frequency output connected tothe secondary of said output transformer, a Wheatstone bridge networkincluding thermistor resistances having negative temperature coefil- Icient of resistance, a second thermionic valve amplifier havingassociated input and output transformers the latter having its primaryconnected in series with the plate circuit of the first thermionicvalve, and having a secondary winding connected to the first pair ofdiagonals of the bridge network, a condenser shunting the lattersecondary and said latter input transformer having a secondary windingconnected to the control grid circuit of said second valve, connectionsfrom the other diagonals of the bridge network to the primary of thelatter input transformer and a signal receiving device connected to asecondary of the latter output transformer.

FRANK FAIRLEY.

RAYMOND WALSH.

